If you're referring to the message that says "Your charger's voltage/wattage cannot be determined. The computer will continue to operate, but at a reduced state of performance" or something similar, what that means is:
A. The charger needs replacing.
B. The motherboard needs replacing.
I had this exact problem several times, Dell laptops seem to be plagued with it. It has something to do with a faulty battery circuitry, I'm not really sure of the specifics. However, basically either:
A. Your computer thinks the battery is full and won't charge it.
B. Your computer thinks it is dangerous to recharge the battery and refuses to do so.
I do suggest you always place your laptop on a flat surface to keep it well ventilated, and to always keep it clean of dust. This can also happen from overheating (but it's unknown to me as to why the battery circuitry would fail.) If replacing the charger does not work, a Dell technician replaced my motherboard and the computer resumed normal operation.
If this is not your problem, please post the error message you are receiving on boot up so we can further diagnose the issue at hand.